Recently, with intensification of exhaust gas regulations, studies for directly measuring the concentration of NO.sub.x contained in the exhaust gases of internal combustion engines for controlling the internal combustion engines or catalysts are on the progress. In particular, studies into a NO.sub.x gas concentration sensor of the type in which oxygen is pumped out by a first oxygen pumping cell, using a solid electrolyte (oxygen ion electro-conductive member), such as zirconia, to an extent that not permits decomposition of NO.sub.x, and in which oxygen is further pumped out of the NO.sub.x containing residual gas by a second oxygen pumping cell for decomposing NO.sub.x and measuring a current Ip.sub.2 generated by the dissociated oxygen ions to detect the NO.sub.x gas concentration, are conducted briskly because it is felt that such sensor can measure the NO.sub.x gas concentration without being affected by obstructive gases contained in the exhaust gas, such as HC or CO.
As a conventional nitrogen oxide concentration sensor, there is disclosed in, for example, SAE paper No. 960334, pages 137 to 142, 1996, a nitrogen oxide concentration sensor in which, in the inside of a laminate structure of solid electrolyte layers of thin plate-shaped zirconia, a first cavity having a first oxygen pumping cell and an oxygen concentration measuring cell, a second cavity having a second oxygen pumping cell, a first diffusion path for communicating the first cavity with the measuring gas side and a second diffusion path for communicating the first and second cavities with each other, are formed, as shown in a plan view of FIG. 3 and in an enlarged cross-sectional view of FIG. 4 taken along line A-A' of FIG. 3.
Referring to FIG. 4, 1 is the first diffusion path, 2 is the first cavity, 3 is the second diffusion path, 5-1, 5-2, 5-3, 5-4, 5-5 and 5-6 are laminated solid electrolyte layers of thin plate-shaped zirconia, 6 is the first oxygen pumping cell, 7 is the oxygen concentration measuring cell, 8 is the second oxygen pumping cell, 9 is an atmosphere inlet and 10 is a heater. The first oxygen pumping cell 6, oxygen concentration measuring cell 7 and second oxygen pumping cell 8 are provided with electrodes of porous platinum or porous rhodium 6-a, 6-b; 7-a, 7-b and 8-a, 8-b, respectively. In the present specification, the oxygen pumping cell and the oxygen concentration measuring cell are termed "oxygen cell".
In the present nitrogen oxide concentration sensor, the oxygen cells are maintained at constant temperatures by supplying a current to the heater 10. The measuring gas, such as an exhaust gas of the internal combustion engines, is introduced via the first diffusion path 1 into the first cavity 2. Oxygen is extracted (pumpted out) by the first oxygen pumping cell 6 from the measuring gas introduced into the first cavity 2. The oxygen concentration in the first cavity 2 is monitored by the oxygen concentration measuring cell 7 so that oxygen is extracted up to a low oxygen concentration at which nitrogen oxides are not decomposed. The oxygen concentration is maintained at the resulting value.
The gas in the first cavity 2, maintained at this low oxygen concentration, is introduced via the second diffusion path 3 into the second cavity 4. From the measuring gas, introduced into the second cavity 4, oxygen is extracted by the second oxygen pumping cell 8. Since oxygen is extracted substantially completely by the second oxygen pumping cell 8 to achieve a lower oxygen concentration, and also since nitrogen oxides are completely decomposed into nitrogen and oxygen under the catalytic function of porous rhodium used as an electrode, oxygen produced on decomposition of nitrogen oxides are extracted by the second oxygen pumping cell 8. It is disclosed that, since the current value (.mu.A) of the current flowing through the second oxygen pumping cell 8 has a linear relation with the concentration (ppm) of the nitrogen oxides contained in the measuring gas, the concentration of nitrogen oxides contained in the measuring gases may be measured if the correlation between the concentration of the nitrogen oxides and the pump current Ip2 is obtained in advance.